Active pull-up circuit

ABSTRACT

An active pull-up circuit for use in a sense amplifier or the like, comprises an enhancement type MIS transistor, a MIS capacitor controlled by a clock signal, and a depletion type MIS transistor controlled by another clock signal (φ 2  &#39;). In this circuit, the two clock signals are bilevel signals having potentials which are the same as potentials of two power supplies.

BACKGROUND OF THE INVENTION

The present invention relates generally to a MIS (Metal-Insulator-Semiconductor) dynamic random access memory and, more particularly, to an active pull-up circuit for use in a sense amplifier, a word decoder or the like.

In general, a MOS (broadly, MIS) dynamic memory device of a one-transistor and one-capacitor type incorporates memory cells, each comprising a capacitor and an enhancement type transistor which serves as a switching gate for charging or discharging the capacitor. In this case, the presence or absence of charges corresponds to the data "0" or "1." Such memory cells are arranged at intersections between word lines and bit line pairs. In addition, dummy memory cells, which are similar in structure to the memory cells, are arranged at intersections between dummy word lines and the bit line pairs.

In the above-mentioned memory device, a sense amplifier is provided for each bit line pair, in order to read memory information. Therefore, when a memory cell is selected, so that a small difference in potential is generated between the bit line pair connected to the selected memory cell, the sense amplifier senses or enlarges the small difference in potential by pulling down the low-level side potential of the bit line pair. As a result, a large difference in potential is obtained between the bit line pair. This large difference is helpful in the read operation. However, it should be noted that, even in this case, the high-level side potential of the bit line pair is also decreased slightly due to the capacity coupling of the bit line, leak currents and the like, thereby deteriorating the sensing speed of the sense amplifier. To avoid this, the high-level potential is again pulled up by an active pull-up circuit.

A first conventional active pull-up circuit for use in a sense amplifier comprises: a first enhancement type MIS transistor having a drain connected to a power supply (V_(cc)) and a source connected to a bit line; a MIS capacitor having an electrode for receiving a first clock signal and another electrode connected to the gate of the first enhancement type transistor; and a second enhancement type MIS transistor having a drain connected to the gate of the first enhancement type transistor, a source connected to the bit line and a gate receiving a second clock signal (See: Digest of Technical Papers of 1980 IEEE International Solid-State Circuits Conference, pp. 230-231, FIG. 1). In this case, the potential of the first clock signal is low (V_(ss)) and high (V_(cc)) during the stand-by mode and the pull-up mode, respectively. On the other hand, the potential of the second clock signal is high (>V_(cc) +V_(th)) and low (V_(cc)) during the stand-by mode and the pull-up mode, respectively. Here, V_(th) is a common threshold voltage of the first and second enhancement type transistors.

However, in the above-mentioned first conventional circuit, it is difficult to generate the second clock signal having a high potential which is higher than V_(cc) +V_(th), since another power supply or a bootstrap circuit is required for generating such a high potential.

A second conventional active pull-up circuit comprises a depletion type MIS transistor instead of the second enhancement type MIS transistor of the first conventional circuit. In this case, it should be noted that the gate of the depletion type transistor is connected to a power supply (V_(ss)) or to ground (See: Digest of Technical Papers of 1979 IEEE International Solid-State Circuits Conference, pp. 142-143, FIG. 1). Therefore, this circuit has an advantage in that a clock signal whose potential is very high (>V_(cc) +V_(th)) is unnecessary.

However, the second conventional circuit has a disadvantage in that a selection range for the negative threshold voltage V_(th) (d) of the depletion type transistor is so small that it is difficult to manufacture a semiconductor device including such active pull-up circuits. This is because the potential of the gate of the first enhancement type transistor is at most |V_(th) (d)| during the stand-by mode, which requires a large bootstrap effect due to the active pull-up circuit and, accordingly, requires a large capacity of the MIS capacitor. Therefore, the value of |V_(th) (d)| is preferably as large as possible. On the other hand, the potential of the high-level bit line prior to the pull-up mode must be higher than |V_(th) (d)|, in order to cause the bootstrap effect of the active pull-up circuit. Therefore, the value of |V_(th) (d)| is preferably as small as possible, which is, however, contradictory to the above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an active pull-up circuit comprising a depletion type MIS transistor which has a wide selection range.

It is another object of the present invention to provide a dynamic memory device comprising sense amplifiers with such active pull-up circuits.

According to the present invention, there is provided an active pull-up circuit comprising: first and second power supplies, the potential of the first power supply being higher than that of the second power supply; an output line; a first means for generating a first clock signal having a potential which is the same as a second potential level during the stand-by mode, while it is the same as a first potential level, which is higher than the second potential level, during the pull-up mode; a second means for generating a second clock signal having a potential which is the same as a third potential level during the stand-by mode, while it is the same as a fourth potential level, which is lower than the third potential level, during the pull-up mode; means, connected to the first power supply and the output line, for pulling up the potential of the output line; means, connected to the first clock signal generating means and the pulling-up means, for supplying the first clock signal to the pulling-up means; a depletion type MIS transistor having a drain connected to the pulling-up means and the clock signal supplying means, a source connected to the output line and a gate connected to the second clock generating means.

According to the present invention, there is also provided a dynamic memory device comprising: first and second power supplies, the potential of the first power supply being higher than that of the second power supply; a first clock signal generating means for generating a first clock signal having a potential which is high during the stand-by mode; a second clock signal generating means for generating a second clock signal having a potential which is the same as a second potential level during the stand-by mode, while it is the same as a first potential level, which is higher than the second potential level, during the pull-up mode; a third clock signal generating means for generating a third clock signal having a potential which is the same as a third potential level during the stand-by mode, while it is the same as a fourth potential level, which is lower than the third potential level, during the pull-up mode; a plurality of word lines; a plurality of pairs of bit lines; a plurality of memory cells, arranged at intersections between the word lines and the bit lines; a plurality of precharging means, each connected to the first power supply, to the first clock signal generating means and to one of the bit lines, for precharging the bit lines; a plurality of sense amplifiers, each connected between one pair of the bit lines, for sensing the difference in potential between the pair of the bit lines during the sensing mode; a plurality of active pull-up circuits, each connected to the first power supply, to the second and third clock signal generating means and to one of the bit lines, for pulling up the potentials of the bit lines; and wherein each of the active pull-up circuits comprises: a pulling-up means, connected to the first power supply and one of the bit lines, for pulling up the potential of the bit line; a clock signal supplying means, connected to the second clock signal generating means and the pulling-up means, for supplying the second clock signal to the pulling-up means; and a depletion type MIS transistor having a drain connected to the pulling-up means and the clock signal supplying means, a source connected to one of the bit lines and a gate connected to the third clock generating means.

The present invention will be more clearly understood from the following description contrasting the present invention with the conventional circuits and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating one conventional dynamic memory device including active pull-up circuits;

FIGS. 2A and 2B are timing waveform diagrams of the signals appearing in the circuit of FIG. 1;

FIG. 3 is a circuit diagram illustrating another conventional dynamic memory device including active pull-up circuits;

FIGS. 4A and 4B are timing waveform diagrams of the signals appearing in the circuit of FIG. 3;

FIG. 5 is a circuit diagram illustrating an embodiment of the dynamic memory device including active pull-up circuits according to the present invention;

FIGS. 6A and 6B are timing waveform diagrams of the signals appearing in the circuit of FIG. 5;

FIG. 7 is a circuit diagram illustrating another embodiment of the word decoder including an active pull-up circuit according to the present invention; and

FIGS. 8A, 8B, 9A and 9B are timing waveform diagrams of the signals appearing in the circuit of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, which illustrates one conventional dynamic memory device including active pull-up circuits (Ref.: the above-mentioned Digest of Technical Papers of 1980 IEEE International Solid-State Circuits Conference), a sense amplifier SA formed by cross-coupled transistors Q₁ and Q₂ is connected to one pair of bit lines BL₁ and BL₂. The sense amplifier SA is activated by a transistor Q₃ which is also common to other sense amplifiers (not shown). A transistor Q₄ is used for equalizing the potentials of the pair of the bit lines BL₁ and BL₂ during the stand-by mode (during the precharge mode).

At intersections between the word lines WL₁ and WL₂ and the bit lines BL₁ and BL₂, memory cells MC₁ and MC₂ which are of the one-transistor and one-capacitor type, are provided. In this case, the memory cell MC₁ (or MC₂) is comprised of a transistor Q₅ (or Q₅ ') and a capacitor C₁ (or C₁ '). On the other hand, at intersections between the dummy word lines DWL₁ and DWL₂ and the bit lines BL₁ and BL₂, a dummy memory cells DMC₁ and DMC₂ are provided. In this case, the dummy memory cell DMC₁ (or DMC₂) is comprised of a transistor Q₆ (or Q₆ '), a capacitor C₂ (or C₂ ') whose capacity is about half of that of the capacitor C₁ (or C₁ '), and a transistor Q₇ (or Q₇ '). In addition, transistors Q₈ and Q.sub. 8 ' are used for precharging the bit lines BL₁ and BL₂, respectively, during the stand-by mode. Further, active pull-up circuits APU₁ and APU_(2l) are connected to the terminals of the bit lines BL₁ and BL₂, respectively.

The active pull-up circuit APU₁ (or APU₂) comprises an enhancement type transistor Q₉ (or Q₉ ') having a drain connected to a power supply V_(cc) which is, for example, 5 volts, a source connected to the bit line BL₁ (or BL₂) and a gate connected to a node N₁ (or N₂); a MOS capacitor C₃ (or C₃ ') having an electrode connected to the node N₁ (or N₂) and having another electrode which receives a clock signal φ₂ ; and an enhancement type transistor Q₁₀ (or Q₁₀ ') having a drain connected to the node N₁ (or N₂), a source connected to the bit line BL₁ (or BL₂) and a gate which receives a clock signal φ₂. In this case, the potential of the clock signal φ₂ is low (V_(ss) =0 volt) and high (V_(cc)) during the stand-by mode and the pull-up mode, respectively. On the other hand, the potential of the clock signal φ₂ is high (>V_(cc) +V_(th)) and low (V_(cc)) during the stand-by mode and the pull-up mode, respectively. (Here, V_(th) is a common threshold voltage of the enhancement transistors.)

FIGS. 2A and 2B are timing waveform diagrams of the signals appearing in the circuit of FIG. 1. Referring to FIGS. 2A and 2B, the operation of the circuit of FIG. 1 will now be explained. First, at a time t₁ when the potential of the clock signal φ₀ is changed from V_(ss) to V_(cc) +V_(th) +α, the transistors Q₄, Q₇, Q₇ ', Q₈ and Q₈ ' are turned on. As a result, the bit lines BL₁ and BL₂ are precharged up to V_(cc), and the capacitors C₂ and C₂ ' of the dummy memory cells DMC₁ and DMC₂ are discharged. It should be noted that, since the potential of the clock signal φ₂ is also V_(cc) +V_(th) +α, the potentials at the nodes N₁ and N₂ become the same as those of the bit lines BL₁ and BL₂, respectively, that is, V_(cc). This is called a stand-by mode or a precharge mode.

Next, at a time t₂, the potential of the clock signal φ₀ is changed from V_(cc) +V_(th) +α to V_(ss) so as to complete the precharge operation. Simultaneously, the potential of the clock signal φ₂ is also changed from V_(cc) +V_(th) +α to V_(cc), so that the transistors Q₁₀ and Q₁₀ ' are turned off.

In addition, at the time t₂, the potentials of the word line WL₁ and the dummy word line DWL₂, which is located on an opposite side of the word line WL₁ with regard to the sense amplifier SA, are changed from V_(ss) to V_(cc) +V_(th) +α, so that the operation enters into a selected mode. As a result, the selected memory cell MC₁ and the dummy memory DMC₂ are connected to the bit lines BL₁ and BL₂, respectively, so that the potentials of the bit lines BL₁ and BL₂ are changed. However, if the capacitor C₁ is charged in advance, the potentials of the bit line BL₁ remains at V_(cc) as indicated by FIG. 2B. In contrast, the potential of the bit line BL₂ is reduced due to the presence of the discharged capacitor C₂ '. Therefore, a small difference in potential between the bit lines BL₁ and BL₂ is generated.

Next, at a time t₃ when the potential of the clock signal φ₁ is changed from V_(ss) to V_(cc), the operation enters into a sensing mode. That is, the transistor Q₃ is turned on so as to cause the sense amplifier SA to be operated. In this case, the transistor Q₂ is turned on so as to pull the potential of the bit line BL₂ down to V_(ss). At this time, since the transistor Q₁ is turned off, the potential of the bit line BL₁ seems to remain at V_(cc) ; however, actually, the potential of the bit line BL₁ is reduced slightly due to the capacity coupling of the bit line BL₁, leakage currents and the like. Such potential reduction is indicated by ΔV in FIG. 2B.

It should be noted that, if the potential reduction ΔV generated on the high-level bit line BL₁ is large, the difference in potential between the bit lines BL₁ and BL₂ becomes so small as to deteriorate the read operation. To avoid such a potential reduction, the potential of the high-level bit line BL₂ is again pulled up to V_(cc) by the active pull-up circuit APU₁.

That is, at a time t₄, the potential of the clock signal φ₂ is changed from V_(ss) to V_(cc), so that the operation enters into a pull-up mode. In the active pull-up circuit APU₁, since the source-gate voltage ΔV of the transistor Q₁₀ is smaller than the threshold voltage V_(th), the transistor Q₁₀ remains in an off-state, so that the capacity coupling of the capacitor C₃ boosts the potential at the node N₁, as illustrated in FIG. 2B, so as to ensure that the transistor Q₉ is turned on. Therefore, the bit line BL₁ is again charged by the current flowing through the transistor Q₉ so that the potential of the bit line BL₁ is returned to V_(cc), as illustrated in FIG. 2B. On the other hand, in the active pull-up circuit APU₂, since the source-gate voltage of the transistor Q₁₀ ' is larger than the threshold voltage V_(th), the transistor Q₁₀ ' is turned on. In addition, the transistor Q₂ is also turned on. As a result, even when the potential at the node N₂ is pulled up by the capacity coupling of the capacitor C₃ ', a current flows through the transistor Q₁₀ ', the bit line BL₂, the transistor Q₂ and the transistor Q₃. That is, the capacity coupling of the capacitor C₃ ' does not boost the potential at the node N₂. Therefore, the transistor Q₉ ' does not conduct and accordingly, the bit line BL₂ is not charged.

Thus, at the end of the pull-up mode operation, the difference in potential between the bit lines BL₁ and BL₂ is V_(cc) -V_(ss), which is helpful for the read operation.

However, in the active pull-up circuit APU₁ or APU₂ of FIG. 1, there is a fatal disadvantage in that the clock signal φ₂, having a potential level which is larger than the power supply voltage V_(cc), requires another power supply or a bootstrap circuit for generating such high potential.

FIG. 3 is a circuit diagram illustrating another conventional dynamic memory device including active pull-up circuits (Ref.: the above-mentioned Digest of Technical Papers of 1979 IEEE International Solid-State Circuits Conference). In FIG. 3, the elements which are the same as those of FIG. 1 are denoted by the same references. That is, in FIG. 3, instead of the enhancement type transistors Q₁₀ and Q₁₀ ' of FIG. 1, depletion type transistors Q₁₁ and Q₁₁ ', each having a gate connected to ground (V_(ss)) are provided. Therefore, the active pull-up circuit of FIG. 3 has an advantage in that the above-mentioned clock signal φ₂ having a high potential level is unnecessary.

FIGS. 4A and 4B are timing waveform diagrams of the signals appearing in the circuit of FIG. 3. Referring to FIGS. 4A and 4B, the operation of the circuit of FIG. 3 will now be explained.

During the stand-by (precharge) mode between the times t₁ and t₂, since the potential of the clock signal φ₀ is V_(cc) +V_(th) +α, the transistors Q₄, Q₇, Q₇ ', Q₈ and Q₈ ' are turned on, so that the bit lines BL₁ and BL₂ are precharged to V_(cc) and the capacitors C₂ and C₂ ' are discharged. However, it should be noted that, in this case, the potentials at the nodes N₁ and N₂ are -V_(th) (d), where V_(th) (d) is a negative threshold voltage of the depletion type transistors Q₁₁ and Q₁₁ '.

During the selected mode between the times t₂ and t₃, the operation of FIG. 3 is similar to that of FIG. 1. However, the potentials at the nodes N₁ and N₂ remain at -V_(th) (d). In addition, during the sensing mode between the times t₃ and t₄, the operation of FIG. 3 is also similar to that of FIG. 1.

During the pull-up mode after the time t₄, the potential of the clock signal φ₂ is changed from V_(ss) to V_(cc). At this time, in the active pull-up circuit APU₁, even when the transistor Q₁₁ is of the depletion type, the transistor Q₁₁ remains in an off-state due to the fact that the potential of the bit line BL₁ is so high that the source-gate voltage of the transistor Q₁₁ is lower than the negative threshold voltage V_(th) (d). Therefore, the capacity coupling by the capacitor C₃ boosts the potential at the node N₁, as illustrated in FIG. 4B, so as to ensure that the transistor Q₉ is turned on. Therefore, the bit line BL₁ is again charged to V_(cc) by the current flowing through the transistor Q₉. On the other hand, in the active pull-up circuit APU₂, since the potential of the bit line BL₂ is low (=V_(ss)), the source-gate voltage of the depletion type transistor Q₁₁ ' is not so negative as to turn off the transistor Q₁₁ '. That is, the transistor Q₁₁ ' is turned on. In addition, the transistor Q₂ is also turned on. As a result, even when the potential at the node N₂ is pulled up by the capacity coupling of the capacitor C₃ ', a current flows through the transistor Q₁₁ ', the bit line BL₂, the transistor Q₂ and the transistor Q₃. That is, the capacity coupling of the capacitor C₃ ' does not boost the potential at the node N₂. Therefore, the transistor Q₉ ' does not conduct and accordingly, the bit line BL₂ is not charged.

However, the above-mentioned active pull-up circuits APU₁ and APU₂ of FIG. 3 have a disadvantage in that a selection range for the negative threshold voltage V_(th) (d) is small, due to the fact that the potential of the gates of the depletion type transistors is fixed at V_(ss).

That is, since the gate potential of the depletion type transistor Q₁₁ is V_(ss) (0 volt), the node N₁ is charged up to |V_(th) (d)| at most. (V_(th) (d) is, for example, -3 volts.) It should be noted that it is necessary for the potential at the node N₁ to be higher than V_(cc) +V_(th) prior to the pull-up mode, in order to pull up the potential of the high-level bit line, which, in this case is BL₁, to V_(cc). In this case, the following should be satisfied:

    V.sub.cc +V.sub.th +α≦|V.sub.th (d)|+V.sub.cc ×k

where |V_(th) (d)| is the precharge potential of the high-level bit line BL₁ ;

V_(cc) ×k: the increase in potential of the high-level bit line BL₁ due to the bootstrap effect by the clock signal φ₂ ; and

k: a constant in consideration of an overdrive quantity, which is represented by C_(L) /(C_(L) +C₃) where C_(L) is the load capacity (the gate capacity of the transistor Q₉, the junction capacities of the capacitor C₃ and the transistor Q₁₁ and so on) of the node N₁, and C₃ represents the capacity of the capacitor C₃. Therefore, it is preferable for the value |V_(th) (d)| to be as large as possible so as to precharge the high-level bit line to a higher level.

On the other hand, prior to the pull-up mode, if the potential of the bit line BL₁ is lower than |V_(th) (d)|, the charges at the node N₁ are discharged through the transistor Q₁₁, so that the potential at the node N₁ falls. At worst, even when the potential of the clock signal φ₂ is raised during the pull-up mode, the transistor Q₉ can not conduct and, accordingly, the pull-up operation for the high-level bit line cannot be performed. To avoid this, it is necessary that |V_(th) (d)|≦V_(BL1)

where V_(BL1) is the potential of the high-level bit line BL₁ prior to the pull-up mode. This requires that the value |V_(th) (d)| be as small as possible, however, this contradicts the above requirement.

Thus, the value |V_(th) (d)| should be determined in consideration of the above, and accordingly, the selection range for the value |V_(th) (d)| is small. Actually, the value V_(th) (d) is, for example, -3 volts.

The above-mentioned limitation on the value |V_(th) (d)| seems to be compensated for by enhancing the bootstrap effect during the pull-up mode. However, in order to enhance the bootstrap effect, it is necessary to increase the capacity of the capacitor C₃ (C₃ ') as a MOS capacitor, or to increase the amplitude of the clock signal φ₂. However, it is not easy to increase the capacity of a MOS capacitor and, in addition, it is disadvantageous to enhance the potential of the clock signal φ₂ to be higher than V_(cc), since the number of power supplies becomes large.

FIG. 5 is a circuit diagram illustrating an embodiment of the dynamic memory device including active pull-up circuits according to the present invention. In FIG. 5, the elements which are the same as those of FIG. 3 are denoted by the same references. In FIG. 5, the depletion type transistors Q₁₁ and Q₁₁ ' are controlled by a clock signal φ₂ ', which is different from FIG. 3. During the stand-by mode, the potential of the clock signal φ₂ ' is high (third potential level) so as to turn on the depletion type transistors Q₁₁ and Q₁₁ ' and, accordingly, to charge the nodes N₁ and N₂ to V_(cc). On the other hand, during the pull-up mode, the potential of the clock signal φ₂ is low (fourth potential level) so as to turn on the depletion type transistor on the low-level bit line and turn off the depletion type transistor on the high-level bit line, even when the potential of the high-level bit line is considerably reduced. Thus, the potential of the clock signal φ.sub. 2 ' is V_(cc) (first potential level) and V_(ss) (second potential level) during the stand-by mode and the pull-up mode, respectively.

FIGS. 6A and 6B are timing waveform diagrams of the signals appearing in the circuit of FIG. 5. Referring to FIGS. 6A and 6B, the operation of the circuit of FIG. 5 will now be explained.

During the stand-by mode between the times t₁ and t₂, the potentials at the nodes N₁ and N₂ are also the same as those of the bit lines BL₁ and BL₂, respectively, that is, V_(cc).

During the selected mode between the times t₂ and t₃ and during the sensing mode, the operation of FIG. 5 is similar to that of FIG. 3. However, note that the potential at the node N₁ which is located on the high-level bit line BL₁ remains at V_(cc).

During the pull-up mode, in the active pull-up circuit APU₁, it is easy to pull up the potential at the node N₁ to be higher than V_(cc) +V_(th), since the potential at the node N₁ is already V_(cc) prior to the pull-up mode. In addition, note that the increase of the potential of the node N₁ due to the bootstrap effect is small, so that the capacity of the capacitor C₃ (C₃ ') is small as compared with FIG. 3.

Thus, the limitation of a selection range for the value |V_(th) (d)|, required by the device of FIG. 3, does not exist. Accordingly, a reference voltage V_(R), for determining whether the potential of the bit lines BL₁ and BL₂ is high or low, can be freely selected by the common negative threshold voltage of the transistors Q₁₁ and Q₁₁ '. In this case, V_(R) is equal to |V_(th) (d)|.

FIG. 7 is a circuit diagram illustrating another embodiment of the word decoder including an active pull-up circuit according to the present invention. In FIG. 7, the elements of an active pull-up circuit APU₃ are the same as those of the active pull-up circuits APU₁ or APU₂ of FIG. 5. A transistor Q₂₃ is used for precharging a node N₂₂ during the stand-by mode, while a transistor Q₂₄ is a transfer gate. A transistor Q₂₅ serves as a word driver which uses a word driver clock signal φ_(WD) so as to drive a word line WL. In addition, transistors Q₃₁, Q₃₂, . . . , Q_(3n) are used for decoding address signals A₀ (or A₀), A₁ (or A₁), . . . , A_(n) (or A_(n)). That is, for example, the gate of the transistor Q₃₁ receives one of the signals A₀ and A₀. Similarly, the gate of the transistor Q₃₂ receives one of the signals A.sub. 1 and A₁. The operation of the circuit of FIG. 7 will now be explained.

FIGS. 8A and 8B are timing waveform diagrams of the signals appearing in the circuit of FIG. 7, in the case where the word line WL is selected. Referring to FIGS. 8A and 8B, at a time t₁ ', the potential of the clock signal φ₀ is changed from V_(ss) to V_(cc) +V_(th) +α and in addition, the potential of the clock signal φ₂ ' is changed from V_(ss) to V_(cc) (from the fourth potential level to the third potential level), as illustrated in FIG. 8A. As a result, nodes N₂₁, N₂₂ and N₂₃ are precharged to V_(cc). In this case, the potentials at the nodes N₂₁ and N₂₂ are V_(cc), while the potential at the node N₂₃ is V_(cc) -V_(th) due to the fact that the gate of the enhancement type transistor Q₂₄ receives V_(cc), as illustrated in FIG. 8B.

Next, at a time t₂ ', the potential of the clock signal φ₀ falls to V_(ss), so that the nodes N₂₁, N₂₂ and N₂₃ enter a floating state. When the word line WL is selected, the potentials of the address signals A₀ (or A₀), A₁ (or A₁), . . . , A_(n) (or A_(n)) are essentially V_(ss) ; however, actually, the potentials of such address signals rise for a short period, as illustrated in FIG. 8A, due to the operation of address buffers (not shown). As a result, the transistors Q₃₁, Q₃₂, . . . , Q_(3n) are also turned on, so that the potential at the node N₂₂ is a little reduced as illustrated in FIG. 8B. Such potential reduction is compensated for by the active pull-up circuit APU₃.

That is, at a time t₃ ', the potential of the clock signal φ₂ rises up to the first potential level, so that the capacity coupling of the capacitor C₂₁ boosts the node N₂₁ and accordingly, the transistor Q₂₁ conducts. As a result, the potential at the node N₂₂ is returned to V_(cc) and after that, the potential at the node N₂₃ rises. Therefore, the transistor Q₂₅ causes the word line WL to be high by inputting the clock signal φ_(WD).

FIGS. 9A and 9B are also timing waveform diagrams of the signals appearing in the circuit of FIG. 7, in the case where the word line WL is not selected. At the end of the stand-by mode operation, the potentials at the nodes N₂₁ and N₂₂ are V_(cc), while the potential at the node N₂₃ is V_(cc) -V_(th). Next, at the time t₂ ', at least one of A₀ (or A₀), A₁ (or A₁), . . . , A_(n) (or A_(n)) is changed from V_(ss) to V_(cc), so that the potential at the node N₂₂ falls to V_(ss) and accordingly, the potentials at the nodes N₂₁ and N₂₃ also fall to V_(ss). Therefore, at the time t₃ ', even when the potential of the clock signal φ₂ rises, the capacity coupling of the capacitor C₂₁ does not boost the node N₂₁ and, accordingly, the transistor Q₂₁ remains nonconductive.

As explained hereinbefore, the active pull-up circuit according to the present invention has an advantage, as compared with the conventional circuit of FIG. 3, in that the negative threshold voltage of the depletion type transistor is freely selected. In addition, a dynamic (active) pull-up operation can be performed easily without increasing the capacity of the MOS capacitors used in the active pull-up circuits and without increasing the amplitude of the clock signal for driving the above-mentioned MOS capacitors. 

We claim:
 1. An active pull-up circuit having a stand-by mode and a pull-up mode, comprising:first and second power supplies, the potential of said first power supply being higher than that of said second power supply; an output line; first means for generating a first clock signal having a potential which is of a second potential level during the stand-by mode, and which is of a first potential level, higher than said second potential level, during the pull-up mode; second means for generating a second clock signal having a potential which is of a third potential level during the stand-by mode, and which is of a fourth potential level, lower than said third potential level, during the pull-up mode; means, connected to said first power supply and said output line, for pulling-up the potential of said output line; means, connected to said first means and said pulling-up means, for supplying said first clock signal to said pulling-up means; and a depletion type MIS transistor having a drain connected to said pulling-up means and said first clock signal supplying means, having a source connected to said output line and a gate connected to said second means.
 2. A circuit as set forth in claim 1, wherein said pulling-up means comprises an enhancement type MIS transistor having a drain connected to said first power supply, having a source connected to said output line, and having a gate connected to said clock signal supplying means.
 3. A circuit as set forth in claim 1, wherein said first clock signal supplying means comprises a capacitor having a first electrode connected to said first means and a second electrode connected to said pulling-up means.
 4. A circuit as set forth in claim 1, wherein said first potential level and said third potential level are the same as that of said first power supply, while said second potential level and said fourth potential level are the same as that of said second power supply.
 5. A dynamic memory device having a stand-by mode and a pull-up mode, comprising:first and second power supplies, the potential of said first power supply being higher than that of said second power supply; first clock signal generating means for generating a first clock signal having a potential which is high during the stand-by mode; second clock signal generating means for generating a second clock signal having a potential which is of a second potential level during the stand-by mode, and which is of a first potential level, higher than said second potential level, during the pull-up mode; third clock signal generating means for generating a third clock signal having a potential which is of a third potential level during the stand-by mode, and which is of a fourth potential level, lower than said third potential level, during the pull-up mode; a plurality of word lines; a plurality of pairs of bit lines; a plurality of memory cells, arranged at intersections between said word lines and said bit lines; a plurality of precharging means, each connected to said first power supply, to said first clock signal generating means and to one of said bit lines, for precharging said bit lines; a plurality of sense amplifiers, each connected between one of said plurality of pairs of said bit lines, for sensing the differences in potential between the one pair of said bit lines during the sensing mode; a plurality of active pull-up circuits, each connected to said first power supply, to said second and third clock signal generating means and to one of said bit lines, for pulling up the potentials of said bit lines, each of said active pull-up circuits comprising:pulling-up means, connected to said first power supply and one of said bit lines, for pulling up the potential of the bit line; clock signal supplying means, connected to said second clock signal generating means and said pulling-up means, for supplying said second clock signal to said pulling-up means; and a depletion type MIS transistor having a drain connected to said pulling-up means and said second clock signal supplying means, having a source connected to one of said bit lines, and having a gate connected to said third clock signal generating means.
 6. A device as set forth in claim 5, wherein said pulling-up means comprises an enhancement type MIS transistor having a drain connected to said first power supply, having a source connected to one of said bit lines, and having a gate connected to said second clock signal supplying means.
 7. A device as set forth in claim 5, wherein said second clock signal supplying means comprises a capacitor having a first electrode connected to said second clock signal generating means and having a second electrode connected to said pulling-up means.
 8. A device as set forth in claim 5, wherein said first potential level and said third potential level are the same as that of said first power supply, while said second potential level and said fourth potential level are the same as that of said second power supply.
 9. A device as set forth in claim 5, wherein the potential of said first clock signal is higher than that of said first power supply during the stand-by mode.
 10. A device as set forth in claim 5, wherein said memory cells are of a one-transistor and one-capacitor type. 